Sensory Nerve Conduction Studies

Sensory nerve conduction studies record the sensory nerve action potential (SNAP) — the summed response of sensory axons activated by electrical stimulation. Because sensory cell bodies sit in dorsal root ganglia, SNAPs are generated entirely by peripheral nerve and are spared in pure radiculopathies (where the lesion is proximal to the DRG). This makes the SNAP one of the most specific markers of peripheral nerve pathology vs nerve root disease. Sensory NCS also detects early small-fiber neuropathy in some forms, distinguishes axonal from demyelinating sensory neuropathies, and is essential for evaluation of focal compressive neuropathies. This page covers the technique, normal values, and the clinical interpretation of sensory NCS.

Antidromic vs Orthodromic Recording

Antidromic (Standard)

• Stimulate the nerve trunk proximally; record at the distal digital sensory fibers.
• Example: stimulate median at wrist, record at index finger.
• Larger SNAP amplitude (sensory axons more concentrated distally).
• Less contamination by motor responses.
• Technically simpler.
• Standard for digital sensory studies.

Orthodromic

• Stimulate at digital sensory fibers distally; record over the nerve trunk proximally.
• Anatomically the natural direction of sensory information flow.
• Smaller, more contaminated SNAP.
• Less commonly used; useful for certain comparison studies.

Recording Setup

• Active electrode over the sensory branch of the nerve being tested (e.g., index finger for median sensory).
• Reference electrode 3–4 cm distal (over the next phalanx).
• Ground electrode between stimulator and recording electrodes.
• Filters: 20 Hz low-frequency, 2 kHz high-frequency.
• Sensitivity: 5–20 μV/div (much higher than motor).
• Sweep speed: 1–2 ms/div.

Measurements

Distal Latency (Peak Latency)

• Time from stimulus onset to peak (or onset) of SNAP.
• Onset latency: more accurate for conduction velocity calculation.
• Peak latency: easier to identify; some labs use this for reporting.

SNAP Amplitude

• Peak-to-peak voltage.
• Measured in microvolts (μV).
• Reflects the number of sensory axons activated.
• Substantially smaller than CMAP amplitude (μV vs mV).

Sensory Conduction Velocity

• Distance between stimulation and recording sites / latency.
• For digital sensory studies: usually estimated from single stimulation site.
• Reported in m/s.

Normal Values (Approximate)

(Lab-specific norms vary substantially.)

Standard Tested Nerves

Upper Extremity

• Median sensory: stim at wrist, record at index (digit 2) or middle finger (digit 3).
• Ulnar sensory: stim at wrist, record at little finger (digit 5).
• Radial sensory: stim over the radial nerve at the dorsal radial wrist, record at the dorsal aspect of the thumb’s dorsal first web space.
• Median palmar mixed: stim at palm, record at wrist (for CTS evaluation).

Lower Extremity

• Sural: stim posterior to lateral malleolus, record over the calf or at lateral leg. Pure sensory branch of the sciatic nerve via the tibial.
• Superficial peroneal: stim anterior to lateral malleolus, record over the dorsum of foot.
• Saphenous: stim at medial knee, record at medial leg (technically challenging; less reliable).

Length-Dependent vs Non-Length-Dependent Patterns

Length-Dependent (Distal Symmetric)

• Most common pattern in peripheral neuropathy.
• Distal symmetric loss: longest fibers affected first.
• NCS findings:
  • Sural responses reduced or absent first.
  • Lower extremity sensory abnormalities before upper extremity.
  • Asymmetric findings are unusual.
• Causes: diabetic neuropathy, chemotherapy-induced, alcoholic, vasculitic (often).

Non-Length-Dependent

• Sensory symptoms in arms and legs without distal predominance.
• Suggests ganglionopathy (Sjögren syndrome, paraneoplastic, idiopathic).
• NCS findings:
  • Asymmetric sensory loss across nerves.
  • Upper and lower extremities affected disproportionately.
  • Median and ulnar may be affected at the same time.

Demyelinating vs Axonal Sensory Patterns

Demyelinating

• Prolonged distal latency.
• Reduced conduction velocity.
• SNAP may be reduced but the pattern is similar to motor demyelinating.
• Less commonly diagnostic alone; sensory NCS is often the entry point that leads to diagnosing CIDP via motor NCS.

Axonal

• Reduced amplitude (most sensitive finding).
• Distal latency normal or mildly prolonged.
• Conduction velocity normal or near-normal.
• Loss of SNAP amplitude precedes prolonged latency in early disease.

Radiculopathy vs Peripheral Neuropathy

This is one of the most important applications of sensory NCS:

• Pure radiculopathy: sensory NCS is NORMAL (lesion proximal to DRG; cell bodies preserved).
• Peripheral neuropathy: sensory NCS abnormal.
• Sensory NCS preserved despite clinical sensory symptoms → think radiculopathy.
• Sensory NCS abnormal → peripheral nerve pathology.

Caveats

• Radiculopathy + concurrent neuropathy: SNAP abnormal due to neuropathy.
• Some myelopathies and CNS sensory pathway disorders: SNAP normal.
• The SNAP is preserved despite numbness in dermatomal patterns of pure radiculopathy.

Focal Compressive Neuropathies on Sensory NCS

Carpal Tunnel Syndrome

• Median palmar sensory study: stim at palm, record at wrist; latency >0.3 ms longer than comparison ulnar palmar = CTS.
• Median digit sensory: prolonged distal latency, reduced amplitude in advanced cases.
• Sensory NCS often the most sensitive finding in mild CTS.

Ulnar Neuropathy at Elbow

• Ulnar dorsal cutaneous sensory branch: tested at the medial dorsum of hand; abnormal in lesions at the elbow but normal in Guyon’s canal lesions.
• Ulnar to digit 5 sensory: reduced amplitude or prolonged latency.

Radial Sensory Neuropathy

• Wartenberg syndrome (radial sensory entrapment at wrist).
• Abnormal radial sensory; preserved radial motor (often).

Meralgia Paresthetica (Lateral Femoral Cutaneous Neuropathy)

• Compressive neuropathy of LFCN as it passes under the inguinal ligament.
• Reduced amplitude of lateral femoral cutaneous sensory NCS (when measurable; often technically challenging).
• Clinical diagnosis often.

Ganglionopathy (Sensory Neuronopathy)

• Pathology at the dorsal root ganglion (cell body).
• Non-length-dependent pattern.
• Loss of all sensory NCS responses including upper and lower extremities.
• Causes:
  • Sjögren syndrome.
  • Paraneoplastic (anti-Hu).
  • Idiopathic sensory ganglionopathy.
  • Chemotherapy-induced (cisplatin, oxaliplatin).
  • Pyridoxine toxicity.
• Distinguished by upper extremity SNAPs reduced/absent in addition to lower extremity.

Small-Fiber Neuropathy

• Affects unmyelinated and small myelinated fibers (C and Aδ).
• Routine NCS (which tests large myelinated fibers) is NORMAL.
• Diagnosis: skin biopsy for intraepidermal nerve fiber density; QSART for sudomotor function; quantitative sensory testing.
• Important: a “normal NCS” does not exclude neuropathy if symptoms suggest small-fiber involvement (burning, neuropathic pain, autonomic symptoms).

Sensory NCS in Diabetic Neuropathy

• Earliest abnormality: reduced sural amplitude.
• Later: sural SNAP absent.
• Then upper extremity involvement.
• Mixed: predominantly axonal pattern but may have demyelinating features in some.

Sensory NCS in Other Common Neuropathies

Vitamin B12 Deficiency

• Combined peripheral and central involvement.
• Sensory NCS may be reduced amplitude (axonal).
• Sometimes more demyelinating features.
• Tabes dorsalis pattern in advanced cases.

Vitamin E Deficiency

• Sensory axonal pattern.
• Lower extremity predominance.

Sjögren Syndrome

• Multiple patterns: small-fiber neuropathy, sensory ganglionopathy, axonal length-dependent, mixed.
• Anti-Ro/SSA antibodies.

CIDP

• Sensory + motor demyelinating.
• SNAPs may be preserved or reduced (variable).

Hereditary Sensory Neuropathies

• HSN/CMT subtypes.
• Variable patterns; often loss of SNAPs early.

Sensory Latency Comparison Studies

Median-Ulnar Palmar Comparison

• Stim at palm, record at wrist for both median and ulnar.
• Latency difference >0.3 ms (median slower than ulnar) supports CTS.
• Sensitive for early CTS even when standard NCS borderline.

Median-Radial Comparison

• Stim at thumb, record both median and radial.
• Difference >0.5 ms (median slower) supports CTS.

Side-to-Side Comparisons

• Asymmetric findings >50% amplitude difference: usually pathologic.
• Compare same nerve bilaterally.
• Useful for focal/unilateral conditions (brachial plexus injury, focal compression).

Special Considerations

Aging

• SNAPs gradually decrease in amplitude with age (more than motor NCS).
• Sural SNAP may be absent in 30%+ of elderly without neuropathy.
• Compare to age-adjusted norms.

Temperature

• Cold limbs slow conduction and reduce amplitude.
• Warm distal limb to 32°C before measuring.

Technical Challenges

• SNAPs are small (μV) and contaminated by motor responses or volume-conducted signals.
• Multiple averages (3–20 sweeps) often needed.
• Clean recording requires good electrode placement and impedance.

🔍 Did You Know?

The clinical principle that “sensory NCS is normal in pure radiculopathy” is one of the most useful diagnostic rules in clinical neurophysiology — and one of the most counterintuitive for clinicians not trained in electrodiagnosis. A patient with classic L5 radiculopathy will report numbness in the lateral leg and dorsum of foot, classically attributed to nerve root compression. Yet when you test the superficial peroneal sensory NCS, the SNAP is preserved at normal amplitude and latency. Why? Because the cell body of the sensory neuron sits in the dorsal root ganglion, OUTSIDE the spinal canal — at the level of the foramen but distal to the disc herniation that compressed the nerve root. Compression of the nerve root proximal to the DRG damages central projections (into the cord) but spares the peripheral axon, leaving the SNAP intact. The clinical implication: a numb patient with a normal SNAP has radiculopathy or a CNS lesion, not a peripheral neuropathy. This finding combined with EMG denervation in the corresponding myotomal muscles confirms the radicular origin. Conversely, a patient with similar symptoms and a reduced sural SNAP has peripheral neuropathy (peripheral axon damaged). The same principle underlies why ganglionopathies (Sjögren, paraneoplastic) produce profound sensory loss with reduced SNAPs (cell body damaged at the DRG itself), while radiculopathies (root compression) leave SNAPs intact. For practicing neurologists, the take-home is that the SNAP localizes the lesion to a precise anatomic location — and “where is the lesion?” is the central question of neurology.

Pitfalls and Pearls

• SNAPs are μV, much smaller than CMAPs (mV): high sensitivity required.
• Antidromic standard: larger SNAP, cleaner recording.
• Sural NCS most sensitive: for length-dependent diabetic and other axonal neuropathies.
• Sural normal: argues against length-dependent neuropathy.
• Non-length-dependent pattern: think ganglionopathy (Sjögren, paraneoplastic).
• Pure radiculopathy: NORMAL sensory NCS (DRG distal to compression).
• Abnormal SNAP with dermatomal numbness: peripheral nerve, not radiculopathy.
• CTS: median palmar mixed sensory most sensitive; median-ulnar comparison >0.3 ms.
• Ulnar dorsal cutaneous: abnormal in elbow lesion; normal in Guyon’s canal lesion.
• Small-fiber neuropathy: NORMAL NCS; need skin biopsy, QSART.
• Cold limbs: slow conduction, reduce amplitude; warm to 32°C.
• Elderly sural: may be absent in 30%+ without neuropathy.
• Side-to-side asymmetry >50% amplitude: usually pathologic.
• Multiple sweep averaging: SNAPs often need 3–20 sweeps for clean recording.
• Diabetic neuropathy: sural amplitude reduces first.
• Vasculitic neuropathy: often asymmetric, multifocal axonal sensory loss.
• Always pair sensory NCS with motor NCS and EMG: complete picture.

References

1. Preston DC, Shapiro BE. Electromyography and Neuromuscular Disorders. 4th ed. Elsevier; 2021.
2. Kimura J. Electrodiagnosis in Diseases of Nerve and Muscle. 4th ed. Oxford University Press; 2013.
3. Aminoff MJ, ed. Aminoff’s Electrodiagnosis in Clinical Neurology. 6th ed. Elsevier; 2012.
4. Mauermann ML, Burns TM. The evaluation of chronic axonal polyneuropathies. Semin Neurol. 2008;28(2):133-151.
5. Lacomis D. Small-fiber neuropathy. Muscle Nerve. 2002;26(2):173-188.